This invention relates to a reactor for the pressure gasification of coal by a treatment with oxygen and water vapour and, if desired, additional gasifying agents at elevated temperatures and under pressures of 5-100 bars, comprising a water-cooled jacket and a rotary grate for moving the material to be gasified and for distributing the gasifying agents introduced into the reactor.
Printed German Specification 1,021,116 discloses a reactor which serves for the pressure gasification of coal and comprises a water-cooled pressure housing, which is provided in its upper portion with an inlet lock and a distributor for the coal to be gasified. The gas producer is provided at its lower end with a rotary grate, which serves to receive and distribute the gasifying agents and to discharge the ash which has been formed. The ash which is discharged by the grate from the reactor shaft is subjected to preliminary disintegration in a crusher and is withdrawn through an ash lock.
Gasification is effected in such reactors at temperatures up to and above 800.degree. C. Air or oxygen-enriched air are often used and pure oxygen is sometimes used as a gasifying agent having an oxidizing activity. Water vapor is used in most cases as a gasifying agent having a reducing activity, which is also required. Carbon monoxide may be used as a further gasifying agent.
To ensure a good gasification performance, a high output and a high efficiency of the pressure gasification reactor, the fuel must be distributed as uniformly as possible throughout the cross-section of the reactor and the entire fuel bed must descend as uniformly as possible. A lead or lag of the fuel at one point or at more points will result in a distortion of the various zones of the material being gasified so that there is only a low degree of combustion. In that case the grate may be endangered by high ash temperatures. Besides, the bed of the material to be gasified may burn through, particularly at the edge of the reactor, so that valuable gases which have been produced may burn with residual oxygen at the edge of the fuel bed or above the same and deflagration may occur.
In pressure gasification reactors of the usual type, the fuel bed rests substantially on a rotary grate, such as is shown in Opened German Specification 2,346,833. The reactors are operated at a fuel rate of 5000-6000 kg/m.sup.2 -h. This high throughput necessitates the provision of a large area for the supply of the gasifying agents because excessively high local temperatures resulting in a rapid sintering or fusing of the ash can be prevented only in this way. The throughput rate required for a pressure gasification reactor is about forty times the throughput rate of a normal gas producer operating under atmospheric pressure.
When the rotary grate is started, the masses of fuel lying on the grate give rise to strong forces. On the other hand, the peripheral zones of the fuel bed which are not disposed directly over the rotary grate are subjected only to the force of gravity for their descent and the action of said force is opposed by the wall friction. For this reason the velocity at which the fuel bed descends at the edge of the reactor is only about 60-80 % of the mean velocity of the descent of the fuel bed as a whole. As the speed of the grate increases, the central portions of the fuel column are preferentially moved and discharged.
Because the pressure gasification reactors have a water-cooled jacket, the fuel near the periphery has not only a larger void ratio but owing to the lower temperatures presents also a lower resistance to the flow of the rising gases than the central portion of the fuel column. As a result, the ash bed quickly grows upwardly in the peripheral zones, and the gases having a high calorific value which leave the central portion of the fuel bed are diluted by gases having a low calorific value from the peripheral portions.
The irregular descent of the fuel column is accompanied by a loss in output because the peripheral fuel portions can be gasified only at a reduced rate. These peripheral fuel portions constitute a major portion of the quantity of the fuel bed.
It has already been attempted to provide a grate which is specially shaped so as to reduce the preferential descent of the central fuel portion. These measures have not given the desired results, for instance, because flights provided on the grate wear off quickly. The movement of the fuel and the distribution of the ash may also be corrected by controlling the feeding of the fuel so as to provide a fuel bed having portions of different height. These measures are highly dependent on the nature of the fuel which is employed and often involve other disadvantages. For instance, the feeding of more closely packed fuel to the peripheral portion of the reactor may have the result that the central portion is too highly permeable to gas temporarily and the fuel burns through in this area. For these reasons, the measures which have been described can be successful only in part and even this success is questionable and depends on the kind of fuel, the particle size distribution of the fuel, the throughput rate of the gas producer and its load conditions. Particularly for caking coals the measures which have been described do not constitute a satisfactory solution. Caking coals cannot be successfully gasified unless all fuel particles are heated up at a uniform rate.